One important area in the technical field of mobile radio communications relates to the handover of User Equipment (UE) from a serving cell to a (neighboring) target cell and the optimization of parameters controlling the handover. In this technical field Self Optimizing Networks (SON) and in particular the failure type and key performance indicator (KPI) specification needed for Mobility Robustness Optimization (MRO) are main targets. A more detailed failure type specification is for instance required for dual threshold measurement events consisting of two dedicated thresholds which have to be fulfilled simultaneously. Such a dual threshold is for instance used for inter-RAT mobility (event B2 in LTE or 3A in UMTS), i.e. handover between different RATs. In this technical field also intra-RAT mobility triggered by dual threshold measurement events (e.g. A5 in LTE) are of interest.
The target of MRO is to optimize those network configuration parameters (e.g. handover (HO) trigger parameters, HO thresholds or timers) such that the number of Radio Link Failures (RLFs) due to problematic HOs is reduced. A HO is triggered by measurement events reported by a UE, wherein the measurement events in turn are triggered by neighbor and serving cell measurements. In case of a simple relative comparison of a signal from a serving Base Station (BS) with a signal from a neighboring potential target BS only a single offset parameter has to be specified for the HO trigger. In case of two different RATs (and sometimes also for two different frequencies in the same RAT) absolute signal values of source and target cells have to be considered, i.e. two thresholds have to be fulfilled simultaneously for triggering a HO event.
Traditionally, the optimization of network configuration parameters in 2G or 3G mobile radio communication networks is based on labor- and cost-intensive drive testing. For a first roll-out, network-wide default configuration parameters are used and if performance management (PM) counters are accumulating RLFs or even call drops in certain service areas, several optimization loops with drive testing equipment are started in order to adapt parameters in a cell-specific manner.
The high costs and the huge effort are the reason for operator's demand for SON mechanisms where cell-specific parameters are autonomously optimized.
As has already been mentioned above there are HOs which are triggered by a measurement event comprising two individual measurements each being related to one of two thresholds, one belonging or being associated to the serving cell of RAT A and the other one belonging to the neighboring or overlaying cell of RAT B. In case of LTE, there is, for instance, the inter-RAT trigger event B2 that is used to determine the point in time of a HO and that is defined as follows:                (1) Serving cell becomes worse than threshold 1 (B2-1)                    AND                        (2) Inter-RAT neighbor cell becomes better than threshold 2 (B2-2)        
The corresponding inter-RAT measurement event on UMTS side, which triggers an inter-RAT HO, is called 3A.
Apart from the possibility that a HO is carried out with a wrong target cell HO problems often result from a bad timing, i.e. the HOs are either initiated too early or too late. For an inter-RAT HO two thresholds have to be simultaneously fulfilled. In case of LTE the above mentioned threshold 1 (B2-1) is compared against the signal strength/quality of the own serving cell (Ms) and the above mentioned threshold 2 is compared against the signal strength/quality of a neighboring cell of different RAT (Mn) and the handover is triggered ifMs<B2-1  (1)ANDMn>B2-2.  (2)
This means that a HO is triggered if the signal strength of the serving communication link (Ms) becomes worse than a threshold related to the serving signal and at the same time the signal strength of a neighboring communication link (Mn) becomes better than a second threshold related to the neighbor signal, i.e. both criteria are fulfilled. The neighboring communication link may be a communication link connecting or belonging to a neighboring cell in the same cellular network and/or a communication link to a mobile radio communication network which relates to another mobile standard.
HO problems which might even cause RLFs may occur in case the thresholds used have an inappropriate value. 3GPP has specified three different categories of HO failure types afflicted by a RLF:    (a) Failures due to too late HO triggering    (b) Failures due to too early HO triggering    (c) Failures due to HO to a wrong cell
For instance in case of LTE inter-RAT mobility the reason for a too late HO (category (a)) can either be a too low threshold B2-1 or a too high threshold B2-2.
There may be a need for improving the mobility robustness of a mobile radio communication network with respect to moving user equipments being supposed to be handed over within the mobile radio communication network from a source cell to a target cell.